Among the current polyethylene catalysts, what is mostly used is still Ziegler-Natta type catalysts. Since coming out in 1950s, Ziegler-Natta type catalyst has developed from 1st generation to 5th generation and has properties ceaselessly improved through researching and developing of various countries' scientists, so that polyethylene industry is propelled. However, with the appearance of new polyethylene processes such as condensed-state technique and super-condensed-state technique, activity of catalysts is required to be higher and higher. Thus at present, one important aspect of researching and developing on this type of catalysts is to enhance activity of a catalyst.
Increasing the atom ratio of magnesium element to transition metal element and increasing the amount of transition metal in catalysts are effective approaches for enhancing activity of catalysts, but there exist operation difficulties during the practice of preparing the catalysts, in particular, conventional impregnating-and-drying processes can hardly meet the requirement. U.S. Pat. No. 6,303,716 discloses a process for preparing a high activity solid catalyst by conventional impregnating-and-drying process, but during practical preparation, when ratio of Mg/Ti reaches above 3.0, catalyst agglomerates will occur. Although the catalyst agglomerates can be removed, yield of the catalyst will be reduced. When a catalyst containing catalyst agglomerates is used in gas phase polymerization, the formation of localized hot-spots in the fluidized bed will likely occur so that reaction temperature control will be more difficult, and catalysts are easily attached to the well of a reactor by static interaction so as to form polymer pieces which, after growing to a certain weight or length and then falling from the well in to the gas distributor of the fluidized bed, can affect operation stability of the fluidized bed.
EP 0771820 discloses a supported catalyst and improved conventional impregnating processes. First, an impregnating solution containing magnesium, titanium and an electron donor is formed, the impregnating solution is mixed with a carrier, and the solvent is evaporated, then the carrier is impregnated with the impregnating solution with same composition once more, and the solvent is evaporated. Impregnating may be repeated as many times as required. As a result, a high ratio of Mg/Ti is obtained, and the resulting catalyst has less agglomerates and high activity. However, the catalyst preparation procedure needs repeated impregnating (at least two times), and it is necessary to strictly control the concentration and volume of the impregnating solution depending on the pore volume of carrier, which renders the preparation procedure quite complicated. Moreover, amount of the electron donor used is greatly increased, therefore, the production cost is increased, and load of solvent recycling unit is also increased.
EP 0584626 discloses a catalyst prepared by reducing TiCl4 using metal magnesium, adding a certain amount of MgCl2 to enhance Mg/Ti ratio, impregnating the resultant solution onto silica and then drying or mixing the resultant solution with a filler and then spray-drying. When spray-drying is employed, Mg/Ti ratio can be controlled within a range of from 4:1 to 6:1, while when impregnating-and-drying process is employed, Mg/Ti ratio can be controlled within a range of from 2:1 to 5:1. The main catalyst component prepared by the process has a structure formula of MgmTiClp[ED]q.